A network is a collection of interconnected nodes that exchange information. The network may be configured as a local-area network (“LAN”) or a wide-area network, such as the Internet. Each network node may be a computer or another device configured to communicate with other nodes on the network. Network nodes typically communicate with one another by exchanging information according to predetermined network communication protocols, or sets of rules defining how information is exchanged between network nodes.
Ethernet is a common network communication protocol used in LANs. One example of Ethernet protocol is set forth in the publicly-available Institute of Electrical and Electronics Engineers (“IEEE”) Standard 802.3, entitled “Carrier Sense Multiple Access With Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications.” The IEEE Standard 802.3 includes, among other things, rules for Ethernet data packet formatting, for different baseband data rates, and for physical transmission media to be used for transmitting Ethernet data packets between network nodes.
As used herein, an “Ethernet mode” is defined as a particular combination of a baseband data rate (i.e., without modulation) and a physical transmission medium. The IEEE Standard 802.3 describes various Ethernet modes including 10BASE-T (“Ethernet”), 100BASE-TX (“Fast Ethernet”), and 1000BASE-T (“Gigabit Ethernet”). 10BASE-T supports baseband Ethernet data transmissions up to 10 megabits per second (“Mbps”) over twisted-pair cables; 100BASE-TX supports baseband transmissions up to 100 Mbps over twisted-pair cables; and 1000BASE-T supports baseband transmissions up to 1 gigabit per second (1000 Mbps) over twisted-pair cables. While 10BASE-T, 100BASE-TX, and 1000BASE-T are popular Ethernet modes in modern LAN architectures, it is apparent that other Ethernet modes can be employed. Accordingly, the 10BASE-T, 100BASE-TX, and 1000BASE-T Ethernet modes are discussed herein by way of example and are not intended to limiting in any manner.
In practice, 10BASE-T and 100BASE-TX LAN connections are typically deployed over a conventional Category-5 cable (“CAT5”) having four pairs of unshielded twisted copper wires. 1000BASE-T connections, however, typically use an enhanced Category-5 cable, or “Category-5e,” cable. Both Category-5 and Category-5e cables typically have 100 ohm impedances and thus require 100 ohm terminations to prevent signal reflections. As used herein, “Category-5 cable” and “CAT5 cable” generally refer to any cable that exhibits the electrical characteristics of a conventional Category-5 or Category-5e cable.
A network node typically includes a network interface card (“NIC”) adapted to transmit and/or receive data. An NIC may contain hardware and software drivers for transmitting data using a selected Ethernet mode. To that end, the NIC may employ line driver circuitry to transmit and/or receive Ethernet data over a physical transmission medium, such as a Category-5 cable.
FIG. 1 illustrates a schematic block diagram of an exemplary Ethernet connection (“link”) 100 having a transmitter side 110 and a receiver side 140 interconnected via a Category-5 cable 130. Transmitter side 110 includes line driver circuitry (e.g., in a first NIC, which may include transmitter side 110 and transformer 120) configured to transmit Ethernet data over cable 130 to receiver side 140 (e.g., in a second NIC, which may include receiver side 140). Transmitter side 110 is coupled to cable 130 and to receiver side 140 via a transformer 120 having a one-to-one turns ratio (1:1). Signals transmitted from transmitter side 110 are coupled through transformer 120 and sent to receiver side 140. The line driver circuitry of transmitter side 110 also includes a pair of 50Ω resistors R1 and R2 that are impedance-matched with effective 50Ω resistances R3 and R4 exhibited by Category-5 cable 130.
The exemplary line driver circuitry shown in FIG. 1 transmits Ethernet data as a differential output signal having a positive output voltage txp and a negative output voltage txn. The resulting Ethernet signal is therefore the difference between positive output voltage txp and negative output voltage txn (i.e., txp-txn). In 10BASE-T Ethernet mode, the positive output voltage txp is typically about 2.2 Volts peak-to-peak (Vpp), and the resulting differential output signal (i.e., txp-txn) is therefore typically greater than 4.4 Vpp.
100BASE-TX and 1000BASE-T modes, however, typically use a lower amplitude differential output signal. In particular, 100BASE-TX and 1000BASE-T modes have a positive output voltage txp and a negative output voltage txn of about 1 Vpp, resulting in a differential output signal of about 2 Vpp. While the 100BASE-TX and 1000BASE-T output signals typically have similar peak-to-peak voltage swings, the IEEE Standard 802.3 specifies that the 1000BASE-T output signal, unlike the 100BASE-TX signal, is encoded using five-level pulse-amplitude modulation for better bandwidth utilization.
Because different networks may employ different Ethernet modes while relying on the same cable for data transmissions, it may be desirable for a NIC to be compatible with multiple Ethernet modes. For example, a device may initially be connected over a 10BASE-T Ethernet link, but may subsequently be connected to a faster 100BASE-TX link. In this situation, the line driver circuitry in the NIC must be capable of transmitting both 10BASE-T and 100BASE-TX Ethernet signals.
FIG. 2 illustrates a conventional multimode Ethernet line driver circuit 200 configured for 10BASE-T and 100BASE-TX/1000BASE-T operations. Ethernet line driver circuit 200 includes voltage sources 210 and 220 for respectively outputting voltages V1 and V2 to generate differential 100BASE-TX and/or 1000BASE-T data signals. Ethernet line driver circuit 200 also includes a second set of voltage sources 230 and 240 for respectively outputting voltages V3 and V4 to generate differential 10BASE-T Ethernet signals. Ethernet line driver circuit 200 further includes a pair of switches 250 and 260 (e.g., n-channel FETs) and a voltage regulator 270 connected to a center tap 280 of transformer 120. Voltage regulator 270 provides a common mode voltage VCM at center tap 280.
During operation in 10BASE-T mode, switches 250 and 260 are turned on and essentially function as short circuits to ground (i.e., low impedances). Voltage sources 210 and 220 are turned off and act as open circuits (i.e., high impedances). Voltage sources 230 and 240 are turned on and provide 10BASE-T Ethernet signals of about 4.4V peak-to-peak. Voltage regulator 270 sets a common mode voltage VCM of about 2.5V at center tap 280. DC current flows from voltage regulator 270 to ground via resistors R1 and R2 (i.e., from center tap 280 to ground), causing voltage drops across resistors R1 and R2. As a result of the voltage drops, the common mode voltage of voltage sources 230 and 240 is reduced and the 10BASE-T Ethernet signals from voltage sources 230 and 240 are prevented from clipping. The voltage drops also result in positive and negative differential output voltages txp and txn of about 2.2 Vpp at the common mode voltage VCM (2.5V). Accordingly, a differential output signal (i.e., txp-txn) having an amplitude of about 4.4 Vpp and at the common mode voltage VCM (2.5V) is produced.
During operation in 100BASE-TX and 1000BASE-T modes, switches 250 and 260 are turned off and essentially function as electrical open circuits. Voltage sources 230 and 240 are also turned off and act as electrical open circuits. Voltage regulator 270 sets a common mode voltage VCM of about 1.25V at center tap 280. Voltage sources 210 and 220 are turned on and provide 100BASE-TX or 1000BASE-T Ethernet signals of about 2 Vpp. Because of the electrical open circuits created by turned-off switches 250 and 260 and turned-off voltage sources 230 and 240, no DC current can flow from voltage regulator 270 to ground. Accordingly, the 100BASE-TX or 1000BASE-T Ethernet signals provided by voltage sources 210 and 220 are centered at the common mode voltage VCM (1.25V). The voltage drops across resistors R1 and R2 result in positive and negative differential output voltages txp and txn of about 1 Vpp at the common mode voltage VCM (1.25V). Accordingly, a differential output signal (i.e., txp-txn) having an amplitude of about 2 Vpp and centered at the common mode voltage VCM (1.25V) is produced.
While Ethernet line driver circuit 200 is compatible with multiple Ethernet modes, it may have some disadvantages in certain applications. For example, two separate and independent driving circuits used respectively for the 10BASE-T mode and the 100BASE-TX and 1000BASE-T mode may add to the cost and complexity of Ethernet line driver circuit 200. In the example illustrated, Ethernet line driver circuit 200 includes essentially two output termination circuits comprising two termination resistors each. In addition, Ethernet line driver circuit 200 may be inefficient because DC current may sink to ground when operating in 10BASE-T mode, as described above. Accordingly, additional power may be consumed by resistors R1 and R2 during operation in 10BASE-T mode.
Therefore, it may be desirable to have a line driver circuit that, in certain applications, may overcome one or more of the disadvantages set forth above.